we are a nuclear reaction
by AkaOkamiRyu
Summary: It happens like a nuclear reaction: one little collision cascading into a mass casualty of the very best variety, unstoppable from the moment it starts until the moment it spends itself out. In which Barry and Caitlin finally spark and burn into something more.
1. I

we are a nuclear reaction, part I

* * *

It happens like a nuclear reaction: one little collision cascading into a mass casualty of the very best variety, unstoppable from the moment it starts until the moment it spends itself out. There are, of course, a multitude of moments that lead to it: two years of build up that create the impetus for that first spark into pure energy, but it's nothing more than a touch, a brief graze of skin, that sends all the potential careening into existence while they are all but helpless to stop it. (Not that they want to, they throw themselves into the fallout fully aware that life in its aftermath will be nothing they've known before).

She's been at the lab all day, first to arrive and last to leave and he's just checking back in before heading out. He honestly hadn't expected anyone to still be there, had been on a run burning some excess energy but really, he shouldn't be surprised. Her back is turned to him, completely focused on the research she's sifting through and completely unaware that she has company, and it pulls a fond smile across his lips as he watches her for a long moment.

He's been doing this more often, pausing in the quiet seconds of the day to breathe her in, and the more he does it the more he wants to (and the more he wonders why it's taken him so damn long to notice, he's always been so smart). He's always known Caitlin is beautiful, but it catches his attention now in ways it never did before. He finds himself mesmerized by the way her curls trail across the bare skin of her neck, the way she chews her lower lip as she sorts through a problem in her data, the way she'll startle and then her lips will curl into a warm smile the second he announces his presence. Her eyes, her mannerisms, her legs, her smile—she's absolutely gorgeous.

And if that were all that it was, he would certainly be able to battle back the flames that these observations incite, but there's so much more to Caitlin Snow than her heart-stuttering appearance: she is brilliant, compassionate, utterly undeterrable and completely uncompromising. In short, everything about her adds fuel to the fire and he's been slowly smoldering for ages now.

Barry shakes away the thoughts, pushes them back into that place where they've been living (and growing and writhing below the surface) steadily for the last several months. Leaning casually against the doorframe, nestling in for the best vantage point for the reaction about to play out, he calls out a quiet "Hey Caitlin," and lets himself laugh aloud when she jumps in just the way he knew she would at the sound of his voice.

Sure enough, as she turns around there's a hand pressed to her chest but a smile on her face that's completely nullifying the scolding look she's attempting to give him. "You scared me," she breathes unnecessarily, though he doesn't mind—a tiny part of him caught up in thinking about better ways to leave her breathless (a tiny part he tries desperately to ignore).

Barry fixes a smile on his face that is insincerely contrite, knowing that Caitlin will read the expression accurately, and pushes himself off the doorframe and into the room. "Sorry about that," and the answering roll of her eyes tells him she knows he isn't. "Didn't actually expect you to still be here." Which is true: he left an hour ago for his run, when Dr. Wells and Cisco were packing up: he'd expected Caitlin to be right behind them, gone long before he'd returned (not that seeing her now is the least bit disappointing).

She just shakes her head, motioning back to the monitor. "Got caught up reading, as usual. But I was just about to head out and grab something to eat, want to join me?"

It's a silly question on several fronts: Barry Allen never turns down a meal and he never turns down a chance to spend some with Caitlin. Whatever else is crackling to life between them, she's one of his very best friends and there's never a dull moment when they're together. "You know I never turn down good food or better company," he teases, making his way over to grab their coats while Caitlin begins the task of shutting off all the monitors and nonessential equipment.

They meet in the middle (like they do on so many things) and tug on their jackets before beginning the short trek out of the lab. "I was thinking Chinese," Caitlin mentions after a moment, glancing up at him to read his expression. Barry can eat pretty much anything at anytime, so he's never exactly fussy about choices, but he still has preferences and she tries to defer to them whenever she can, even if he makes no demands (he's so damn selfless all the time, she nearly needs to trick him into thinking about himself every now and then—it would probably be more obnoxious if it wasn't also so endearing).

"Mmm," he smiles, knowing she'll be watching for it. "Take out and a movie?" It's become one of his favorite ways to spend an evening, stretched lazily across Caitlin's couch (sometimes with Cisco, sometimes without) eating straight from the container, watching Caitlin's nose wrinkle slightly as she accepts that he's not going to use a plate (and heckling her until she eventually caves, desperately hiding her smile as she plucks a dumpling straight from his box with her chopsticks) while a movie plays out on the screen of her television.

"If that's okay with you?"

It's a question she really shouldn't need to ask; he needs to get them to the point where she doesn't ask. As brilliant as she is, he thinks she should see how desperate he always is to spend time with her, how gladly he accepts any chance, but she never sees that about herself and it's frustrating and endearing and he's determined to change it someday. "Sounds like a perfect night to me," he settles on, bumping her shorter shoulder with his own. Contact is Caitlin's language, she works in science and resolutes and he knows it's harder to argue with what's physically tangible so he finds himself backing all his reassurances with a nudge, a hug or a brush of his arm. He also tries to tell himself it's not just an excuse to touch her, but he's not sure which theory is more plausible.

Either way, her smile is radiant even as she rolls her eyes, incorrectly assuming that he's just being overly dramatic. "Good thing you're so easy to please."

And he is, really truly is, as long as the thing doing the pleasing is Caitlin Snow (and isn't that an image he'll never get out of his mind?).

* * *

This will be a two-parter, which I'll be posting in the next few days. Part two will definitely be rated mature but I wanted to separate the scene leading up to it. (Which, by the way, this will be my first mature story so I'm a bit nervous, which is the other reason I wanted to split them up).

Hope you guys liked the start. As always, feedback is appreciated!

Take Care &amp; Best Wishes,

A.O.R.


	2. II

**we are a nuclear reaction, II**

* * *

Caitlin doesn't even bother with plates this time—Barry shoots her a grin without commenting, but knows she catches its meaning anyway. Instead, as soon as they arrive back at her apartment, she spreads the takeout containers across her coffee table and sends Barry to her kitchen for forks and drinks (and if there's a little thrill at the way he can navigate the intricacies of her apartment without any help, well, that's her secret to keep). He comes back a few minutes later with two forks and two glasses of ice water, a wedge of lemon stuck in the rim of the glass he hands her and they settle in.

Barry opens containers while Caitlin flips through her Netflix queue, the living room slowly filling with the scent of hot food and the sound of advertisements until the familiar sounds of a movie's intro replace the ads and Barry looks up laughing. "Aren't you sick of that one?"

Beaming, Caitlin snatches up the container of fried rice. "Not even close," she says as the movie's introduction begins, the voice of a heartbroken Christian Bale filling the living room. They've definitely watched this movie at least 3 other times, but Caitlin loves it—the story is just the right mix of romantic and tragic, the directing is excellent and the music is made all the better for Barry's habit of singing along (during the movie and for days afterwards, whenever she catches him working on something, lost in thought).

By the time they get to Elephant Love Medley, they're both stuffed. Caitlin gets up to put away the leftovers and Barry immediately stands to help. They work together easily, belting along to the respective parts of the song before settling back onto the couch, curled into what has slowly become a familiar position: Barry pressed into the cushions, angled half on his back and half on his side, Caitlin stretched against him, her head tucked up against his clavicle. Over the last year or so, the casual, grounding touches have become more and more common, to the point where they hardly give it a second thought.

Until suddenly they do.

The movie ends with them both half asleep, blinking away a long day as the return of the Netflix menu jars them into reality. Caitlin sits up, followed by Barry, and they both stretch and shake away the vestiges of sleep that are nipping behind both their eyelids. A few long moments pass in this silent dance of regaining consciousness before Barry stands up and starts to move toward the door. "Guess I should get going, work tomorrow," he mumbles through a yawn, leaning over to slip on his sneakers while Caitlin walks over to say goodbye and lock the door.

"Yeah," Caitlin trails off as they stand at an impasse in the doorway, Barry's hand lingering on the knob, half inside and half outside her apartment. Like she usually does, Caitlin leans up to hug him, her fingers dancing across the back of his neck before wrapping him up tight in an embrace he easily returns (he's so good at affection that she wonders how she never realized how much she craved it before him).

It's when they're parting—slowly, lingeringly—that the whole world tips on its axis, the inevitable implosion finally sparking into glorious existence.

Caitlin's fingers brush against the hair at the base of his neck, then stutter and drag along the soft skin of his collar bone as they make their descent back to her side but their trek is interrupted by the shiver that courses down Barry's spine at the contact. Startled, her eyes fly up to his just in time to watch the way his pupils dilate and his gaze zeroes in on her lips. It's her turn to shiver, her fingers reversing their trail as they move simultaneously together, lips meeting in a heated kiss that has been building through months of casual touches and not-strictly-friendly gestures.

Barry slams her front door shut again in his eagerness to get his hands securely on her hips and pull her close but neither really notices the sound. Caitlin's hands are buried in his hair, igniting trails of fiery heat that seep down both of them, all the way to their toes. After what feels simultaneously like forever and yet not nearly long enough, they pull apart gasping, foreheads pressed together and, when they finally flutter open, their eyes locked.

They've had a million conversations over the past few years without saying anything, but this feels like one of those moments where even if words aren't necessary, he probably should to use them so Barry throws it all on the line, wondering if this is doomed to fizzle out or implode into pure energy (he's desperately hoping for the latter). "Caitlin?" It's the only thing he can manage to articulate because there's so much riding on this but it's enough. Like she always seems to, Caitlin knows just what he's asking.

"Yes," and the tone is certain, no question lingering in her voice and that's all he needs to lean in again, slanting his lips to hers and barely choking back a moan when her mouth opens immediately beneath his. He can't hold the guttural sound back though when, a moment later, Caitlin's tongue finds his and it's like his whole system fries in that moment. His grip on her hips tightens and, with a speed only the Flash could manage, they're turned around and Caitlin's back is pressed against the heavy door of her apartment.

Eventually his lips abandon hers in favor of less explored (though thoroughly enjoyed) territory, trailing down her jaw and her neck, his tongue dancing across the column of her throat in a way that causes Caitlin to arch against him. One bare foot curves up to drag across the back of his calf, tugging him just a little closer and the way it causes her legs to part around his is the most intoxicating thing Barry Allen has ever experienced (impressive, considering Caitlin sometimes cooks up super booze just because she gets bored). At least, it's the most intoxicating thing he's experienced until Caitlin lets out this whimpering little moan the moment he presses into her, one hand sliding down her thigh to bring her leg up around him and the friction and the noise and the heat of her through too many layers of clothes makes him feel like he's going absolutely crazy.

"God, Caitlin," he groans between breaths, feeling more turned on than he's ever been, rutted up against her front door, pressing open mouthed kisses against the exposed flesh of her neck and shoulder.

Her fingers, surprisingly gentle and a complete counterpoint to the way her leg is desperately pulling him closer (chasing a sensation that is so good but not nearly enough), guide his mouth back to hers for a less frenzied kiss, which lets them both catch a little air. Still, she's pausing between every other word when she speaks. "We should…" she trails off for another drag of air and in the seconds that takes, he becomes terrified that she's going to say stop, "probably not" his hear stutters painfully in his chest, "do this against my front door." And then it starts beating again, a terribly boyish grin (if a little feral with desire) lighting up his face (thank God. Thank God. Thank God.).

If it's possible, that expression just makes her want him all the more—because does he really think she's changing her mind after all of this? Every nerve in her body feels like it's on fire and there is no way anything but Barry Allen is going to satisfy the ache that's burning through her.

"You're right," he breathes, catching her in another lingering kiss that's more affection than heat (it melts her either way). "That's definitely not how I pictured this."

The quirk of her eyebrow and playful grin on her lips gets his attention right away but it's the challenging, teasing tone that sends a shock to his already throbbing erection. "Oh? And how precisely, Mr. Allen, did you picture this happening?" (She kind of wants to ask how often but she's not sure she can handle what that answer might do to her).

He laughs outright, warm and dizzy and impossibly turned on. Sliding his other hand down from where it's still gripping her hip, Barry hauls her up against him and delights at the way her legs wrap automatically around his waist. "You know me Dr. Snow, I'm more of an act first kind of guy—I'd rather show then tell," and then he's using his particle accelerator induced muscles to walk them both into her bedroom.

He drops her into a sitting position of the foot of her bed, then leans over to drag his fingertips along the hem of her shirt, one last hesitant gesture to defer to Caitlin the right to change her mind before they move unstoppably forward. She threads her arms around his neck and pulls him in for a kiss that has them both sprawled across her comforter and scooting themselves farther up the mattress. This time it's Caitlin who leaves a trail of kisses and nips against his throat, pausing to moan low in his ear when she feels the hard outline of his cock rub against her hip.

She abandons her previous task to tug off his shirt, shimmying out of her own a moment later and rolling her eyes at the way his linger on her mostly uncovered chest. "You've seen them before," she laughs, which causes a look of utter indignance to skirt across his face.

"I did not peek," he maintains as he's done for years, reaching out to trail his fingertips momentarily along the swell of her breasts before they sneak between her back and the bed to unclasp her lacy red bra (of all colors, always the damn red). It winds up somewhere on her floor and he's a little bit proud that he has Caitlin Snow turned on enough to not care that there are clothes strewn about her bedroom haphazardly (or he would be, anyway, if he wasn't so decidedly distracted himself). As if making up for the looks he once did not take, Barry leans down immediately to press a kiss in the space between her breasts, letting his mouth trail up the swell and across the nipple of one, then the other. The sounds she makes, the way Caitlin writhes up against him, rubbing at his painfully constricted cock, only drives him further and he spends the next few minutes lavishing her chest with attention.

He could probably stay like that all night, his teeth wrapped around one nipple and his hand slowly kneading the other, but Caitlin grows impatient beneath him, making a sound somewhere between a needy whine and a growl before reaching for his belt. Before he can drag his mouth away from the fluttering kisses he's teasing her with, she has it undone, is dragging down his zipper and then the palm of her hand is sliding against him and Barry is seeing stars.

Thoroughly convinced, he toes off his sneakers and helps her shimmy off his jeans before he begins his own battle with her pants (because of course, today of all days, she decides to wear something other than a dress or skirt to work). They're a little harder to get off, since Caitlin's either lying on the bed or arching her body flush against his but Barry Allen saves Central City on a regular basis and so is way too stubborn to lose a battle to a pair of pants. Once her pants have joined the growing disarray on the floor, they don't linger long in just her panties or his boxes, enticed by the prospect of seeing one another completely bare.

And then suddenly they are: both completely naked, Caitlin sprawled out on her bed with Barry perched on his arms above her. There's a long measure of near silence (broken only by their breathing) as they take one another in. Nothing they see is really a surprise (they know each other so well, have helped patch each other up often enough, have hugged a thousand times) and yet it's so much more than they imagined (which they've both done more times then they'll likely admit).

Barry breaks the silence first, leaning down to kiss her slowly, smiling like a love struck teenager the moment he pulls away. "God, you are the most beautiful thing I've ever seen." And she flushes, red from her cheeks down to her chest, which is the most adorable sight he never imagined he'd get to see. "Seriously Cait, you're gorgeous." He repeats, kissing her lightly on the lips again before beginning a slow descent down her body, pressing his lips against her every few inches to whisper some new bit of praise that sounds like a prayer against her skin.

For her part, Caitlin just smiles and watches him with eyes that are far softer than they probably should be, but she doesn't care. It's only when he kisses just above her belly button that she realizes what he's planning and snakes her hands down to guide his mouth back to hers. It starts with a feather light kiss but she can't resist tugging his lip with her teeth and hooking one leg around his thigh, pulling him impossibly close to right where she wants him. He can feel the heat as his cock rubs against her lips and they both groan into the kiss. "I like that," she mutters suddenly, and the look on his face makes her laugh because she knows what he's thinking about. "Not that," she breathes, though she arches her hips and rubs against him again to assure Barry that, yes, of course, she likes that too, the dummy. "I mean, I like when you call me Cait."

He grins that ridiculous Barry Allen grin that has been making her fall for him for longer than she even realizes. "Good," he answers, pressing a quick kiss to the corner of her mouth. "Because I like calling you Cait and I plan to do it for a long time to come."

It's the certainty of the statement that puts her completely over the edge, leaving her reaching haphazardly across the bed for her nightstand. He has no idea what she's doing and it shows in the look on his face. Caitlin rolls her eyes. "Barry Allen, you cannot go around saying things like that and then not expecting me to want you, right here, right now. So put those powers to good use and get a damn condom from the drawer."

She sounds needy and bossy and if it was possible to be any more turned on then he's been so far, he is. "Have I mentioned how perfect you are?" Barry asks as he reaches over, opening the top drawer (the pieces have suddenly clicked easily together) and tugging out a condom. Caitlin takes it from him, tearing open the package, wanting the pleasure of rolling it over his hard cock herself.

There's not much talking that follows that, the few words are largely incoherent—ranging from desperate shouts to whispered prayers against warm, sticky skin but the slide of skin against skin, the way they move together, finding rhythms that range from easy and soft to frenzied and hard as the night wears on say an awful lot on their own.

* * *

It happens like a nuclear reaction: one little collision cascading into a mass casualty of the very best variety, unstoppable from the moment it starts until the moment it spends itself out. There are, of course, a multitude of moments that lead to it: two years of build up that create the impetus for that first spark into pure energy, but it's nothing more than a touch, a brief graze of skin, that sends all the potential careening into existence while they are all but helpless to stop it. (Not that they want to, they throw themselves into the fallout fully aware that life in its aftermath will be nothing they've known before).

* * *

So I don't think it's too too far out there on the smut scale, but I've never, ever written a mature fic before so I'm going to say I'm fairly pleased with the result nonetheless. I hope you guys enjoyed it, I know it was a blast and a nice challenge to write. I've had some requests about continuing in this universe, which I think I'd like to try but I have no timeline on when that will happen, so if (likely when) it does, let's just count it as a happy surprise : )

As I said, this is my very first mature/adult fic so any feedback would be greatly appreciated.

Take Care and Best Wishes,

A.O.R.


End file.
